1. Field of Invention
The invention relates to electro-optical sensing devices for detecting the presence or concentration of an analyte in a liquid or gaseous medium. More particularly, the invention relates to (but is not in all cases necessarily limited to) optical-based sensing devices which are characterized by being totally self-contained, with a smooth and rounded oblong, oval, or elliptical shape (e.g., a bean- or pharmaceutical capsule-shape) and an extraordinarily compact size which permit the device to be implanted in humans for in-situ detection of various analytes.
2. Background Art
U.S. Pat. No. 5,356,417, the disclosure of which is incorporated herein by reference, describes a fluorescence-based sensing device comprising indicator molecules and a photosensitive element, e.g., a photodetector. Broadly speaking, in the context of the field of the present invention, indicator molecules are molecules one or more optical characteristics of which is or are affected by the local presence of an analyte. In the device according to U.S. Pat. No. 5,517,313, a light source, e.g., a light-emitting diode ("LED"), is located at least partially within a layer of material containing fluorescent indicator molecules or, alternatively, at least partially within a wave guide layer such that radiation (light) emitted by the source strikes and causes the indicator molecules to fluoresce. A high-pass filter allows fluorescent light emitted by the indicator molecules to reach the photosensitive element (photodetector) while filtering out scattered light from the light source.
The fluorescence of the indicator molecules employed in the device described in U.S. Pat. No. 5,517,313 is modulated, i.e., attenuated or enhanced, by the local presence of an analyte. For example, the orange-red fluorescence of the complex tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) perchlorate is quenched by the local presence of oxygen. Therefore, this complex can be used advantageously as the indicator molecule in an oxygen sensor. Indicator molecules whose fluorescence properties are affected by various other analytes are known as well.
Furthermore, indicator molecules which absorb light, with the level of absorption being affected by the presence or concentration of an analyte, are also known. See, for example, U.S. Pat. No. 5,512,246, the disclosure of which is incorporated by reference, which discloses compositions whose spectral responses are attenuated by the local presence of polyhydroxyl compounds such as sugars. It is believed, however, that such light-absorbing indicator molecules have not been used before in a sensor construct like that taught in U.S. Pat. No. 5,517,313 or in a sensor construct as taught herein.
In the sensor described in U.S. Pat. No. 5,517,313, the material which contains the indicator molecules is permeable to the analyte. Thus, the analyte can diffuse into the material from the surrounding test medium, thereby affecting the fluorescence of the indicator molecules. The light source, indicator molecule-containing matrix material, high-pass filter, and photodetector are configured such that fluorescent light emitted by the indicator molecules impacts the photodetector such that an electrical signal is generated that is indicative of the concentration of the analyte in the surrounding medium.
The sensing device described in U.S. Pat. No. 5,517,313 represents a marked improvement over devices which constitute prior art with respect to U.S. Pat. No. 5,517,313. There has, however, remained a need for sensors that permit the detection of various analytes in an extremely important environment--the human body. Moreover, further refinements have been made in the field, which refinements have resulted in smaller and more efficient devices